The porosity of electrolytic nickel-phosphorus coatings of different compositions was determined, and compared to that of Watts' nickel electrodeposits. Notwithstanding that the current efficiency of nickel-phosphorus plating is quite low, the porosity of thin amorphous nickel-phosphorus coatings containing 17 wt% phosphorus is much lower than that of Watts' nickel. Thin nickel-phosphorus electrolytic coatings containing 8 to 14 wt% phosphorus having a combined amorphous-crystalline structure are, on the contrary, rather porous.